Caprolactam (CPL) is an important raw material for synthesizing chemical products, such as nylon 6, nylon 6.6, etc, and plays a significant role in the modern organic chemical industry. Caprolactam is commercially produced by a liquid phase Beckman rearrangement of cyclohexanone oxime in the presence of fuming sulfuric acid. The mixture solution containing fuming sulfuric acid and caprolactam after the rearrangement reaction is neutralized with ammonium hydroxide to give ammonium sulfate. Ammonium sulfate and crude lactam containing impurities are separated. The crude lactam is extracted with organic solvents, and then it is further purified to recover caprolactam.
FIG. 5 is a schematic diagram illustrating a conventional system for recovering caprolactam from a rearrangement mixture of cyclohexanone oxime and sulfuric acid in common industrial preparations. First of all, the mixture solution containing fuming sulfuric acid and caprolactam obtained after the Beckman rearrangement reaction, and ammonium hydroxide are fed into a neutralization unit 600 via lines 60 and 61, respectively, to carry out neutralization. The crude lactam solution containing impurities is fed into a buffering unit 610 via a line 62, and then it is continuously fed into an extraction unit 630 via a line 63. The organic solvent placed in an organic solvent storage tank 620 is continuously fed into the extraction unit 630 via a line 64, so as to extract the crude lactam solution. The organic and inorganic phases are discharged as effluent from the extraction unit 630, while the lactam solution containing caprolactam is fed into a recovery unit 640 via a line 65 to undergo subsequent purification steps, and caprolactam is recovered.
Taiwanese Patent No. 408109 discloses a method for recovering caprolactam from a rearrangement mixture after neutralization. The rearrangement mixture containing an aqueous caprolactam solution and an aqueous ammonium sulfate solution is extracted with organic solvents in a mixer, and then it is separated into an organic phase and an aqueous phase in a separator. Afterwards, caprolactam is recovered from the organic phase. However, due to an excessive amount of impurities contained in the cyclohexanone oxime raw material or incomplete initial reactions of cyclohexanone oxime, a low grade crude lactam solution may be formed, thereby causing an inseparable third phase between the organic phase and the aqueous phase to be formed during subsequent extractions using organic solvents. In this case, a continuous extraction device can't be smoothly operated, and thus inorganic solution and impurities accompanying the lactam solution all overflow into the subsequent purification steps, resulting in drastically increasing purification loadings, decreasing separation efficiency and raw material consumption. Continuous operation process is further interrupted and even stopped.
Therefore, it is desired to provide a method for recovering caprolactam with high separation efficiency, low raw material consumption, low purification loadings and a continuous operation process.